1. Field of the Disclosure
Generally, the subject matter disclosed herein relates to integrated circuits, and, more particularly, to transistors having strained channel regions by using an embedded strained semiconductor material within the active region to enhance charge carrier mobility in the channel region of a MOS transistor.
2. Description of the Related Art
Modern integrated circuits typically comprise a great number of circuit elements on a given chip area which are positioned and connected to each other according to a specified circuit layout. Transistors as active elements, i.e., as circuit elements enabling signal amplification and signal switching, represent one of the dominant components of an integrated circuit and, therefore, the overall performance of integrated circuits is significantly determined by the performance characteristics of the individual transistor elements. The operational behavior of the transistors, in turn, may depend on the overall dimensions, the basic transistor configuration, the manufacturing techniques used and the like. Generally, a plurality of process technologies are currently practiced, wherein, for complex circuitry, such as microprocessors, storage chips and the like, CMOS technology is currently one of the most promising approaches due to the superior characteristics in view of operating speed and/or power consumption and/or cost efficiency. During the fabrication of complex integrated circuits using CMOS technology, millions of field effect transistors, i.e., N-channel transistors and P-channel transistors, are formed on a substrate including a crystalline semiconductor layer. A MOS transistor or field effect transistor, irrespective of whether an N-channel transistor or a P-channel transistor is considered, comprises so-called PN junctions that are formed by an interface of highly doped drain and source regions with an inversely or weakly doped channel region disposed between the drain region and the source region.
The conductivity of the channel region, i.e., the drive current capability of the conductive channel, is controlled by a gate electrode formed near the channel region and separated therefrom by a thin insulating layer. The conductivity of the channel region, upon formation of a conductive channel due to the application of an appropriate control voltage to the gate electrode, depends on the dopant concentration, the mobility of the charge carriers and, for a given extension of the channel region in the transistor width direction, on the distance between the source and drain regions, which is also referred to as channel length. Hence, in combination with the capability of rapidly creating a conductive channel below the insulating layer upon application of the control voltage to the gate electrode, the conductivity of the channel region per unit length substantially determines the performance of the MOS transistors. Thus, the reduction of the channel length, and associated therewith the reduction of the channel resistivity per unit length in the transistor width direction, renders the channel length a dominant design criterion for accomplishing an increase in the operating speed of the individual transistors and thus of the entire integrated circuit.
The continuing shrinkage of the transistor dimensions, however, involves a plurality of issues associated therewith, such as reduced controllability of the channel, also referred to as short channel effects, and the like, that have to be addressed so as to not unduly offset the advantages obtained by steadily decreasing the channel length of MOS transistors. Since the continuous size reduction of the critical dimensions, i.e., the gate length of the transistors, necessitates the adaptation and possibly the new development of highly complex process techniques, for example, for compensating for short channel effects, it has been proposed to also enhance the channel conductivity of the transistor elements by increasing the charge carrier mobility in the channel region for a given channel length, thereby offering the potential for achieving a performance improvement that is comparable with the advance to a future technology node while avoiding or at least postponing many of the problems encountered with the process adaptations associated with device scaling.
One efficient mechanism for increasing the charge carrier mobility is the modification of the lattice structure in the channel region, for instance by creating tensile or compressive stress in the vicinity of the channel region to produce a corresponding strain in the channel region, which results in a modified mobility for electrons and holes, respectively. For example, for a standard crystallographic orientation of the basic silicon layer, compressive strain in the channel region may increase the mobility of holes, thereby providing the potential for enhancing the performance of P-type transistors. On the other hand, the creation of tensile strain in the channel region of an N-channel transistor may increase electron mobility. The introduction of stress or strain engineering into integrated circuit fabrication is an extremely promising approach for further device generations, since, for example, strained silicon may be considered as a “new” type of semiconductor material, which may enable the fabrication of fast, powerful semiconductor devices without requiring expensive semiconductor materials, while many of the well-established manufacturing techniques may still be used.
Therefore, in some approaches, the hole mobility of PMOS transistors is enhanced by forming a strained silicon/germanium layer in the drain and source regions of the transistors, wherein the compressively strained drain and source regions create uniaxial strain in the adjacent silicon channel region. To this end, the drain and source extension regions of the PMOS transistors are formed on the basis of ion implantation. Thereafter, respective side-wall spacers are formed at the gate electrode as required for the definition of the deep drain and source junctions and the metal silicide in a later manufacturing stage. Prior to the formation of the deep drain and source junctions, these regions are selectively recessed based on the sidewall spacers, while the NMOS transistors are masked. Subsequently, a highly in situ doped or an intrinsic silicon/germanium layer is selectively formed in the PMOS transistor by epitaxial growth techniques. Since the natural lattice spacing of silicon/germanium is greater than that of silicon, the epitaxially grown silicon/germanium layer, adopting the lattice spacing of the silicon, is grown under compressive strain, which is efficiently transferred to the channel region, thereby compressively straining the silicon therein. This integration scenario results in a significant performance gain of the P-channel transistors. Hence, a similar concept has been proposed for N-channel transistors by using a silicon/carbon material that has a smaller lattice spacing compared to silicon.
In other approaches, stressed dielectric materials may be positioned close to the transistor structures such that the internal stress of these materials may act on the transistor and finally create a respective strain in the channel regions. For this purpose, sophisticated deposition and patterning techniques may be applied to form the highly stressed materials in the contact level.
Although the above-described techniques provide enhanced performance characteristics, significant efforts may have to be made, for instance, performing selective epitaxial growth techniques for enhancing the P-channel transistor performance by an embedded silicon/germanium alloy. In other cases, in addition to complex patterning and deposition regimes for providing the highly stressed dielectric materials adjacent to the transistors, the stress transfer mechanism provided by these materials may have to “act” through the various transistor components, such as spacer elements, gate electrodes, metal silicide regions and the like, thereby reducing the overall efficiency. Furthermore, upon aggressive device scaling, the capability for the deposition and patterning of the stressed dielectric materials may be restricted by the device geometry.
The present disclosure is directed to various methods and devices that may avoid, or at least reduce, the effects of one or more of the problems identified above.